Pod drive systems, for propelling and steering marine vessels, typically comprise of one or more pod drive units wherein, as illustrated in FIG. 1, each pod drive unit 2A of a pod drive system 2 typically includes an inboard engine 2B which drives a drive shaft 2C that, in turn, drives an inboard transmission unit 2D that is connected to and drives an underwater steerable gearcase 2E that is rotatably mounted through the hull 2F and supports and drives a propeller 2G. As generally indicated in FIG. 1, engine torque is transmitted from a generally horizontal drive shaft 2C, through a first bevel gear assembly 2H, to a generally vertical arranged intermediate drive shaft 2I extending downwardly through inboard transmission unit 2D to the steerable gearcase 2E. The engine torque of the vertical intermediate drive shaft 2I is, in turn, transmitted through a second bevel gear assembly 2J to a propeller shaft 2K which, in turn, supports and drives a propeller 2G. The pod drive unit 2A allows the propeller 2G to be rotated in the generally horizontal plane, about a steering axis 2L, and through an angular range of, for example, up to 360°, so that the pod drive unit 2A combines and forms both the vessel propulsion function as well as the steering function. The selection of the appropriate maximum starboard and port steering angles will depend on the desired steering performances and design constraints and choices, such as the type of vessel, the design and characteristics of the vessel hull and the desired manoeuvring characteristics.
Pod drive systems, also referred to as azimuthing propulsion systems or azimuth thrusters, have become popular and common in vessels of all sizes for a number of real and perceived advantages. For example, pod drive systems are typically more compact than and offer greater manoeuverability than systems having inboard engines or non-steerable propellers and rudders and are better protected from damage and offer greater manoeuverability than outboard drive systems and many propeller and rudder systems.
However, pod drive systems present a number of problems. Pod drive systems, of various configurations, are used in a wide range of marine vessels ranging from small pleasure craft to large work vessels, such as commercial fishing vessels, and even large ships, such as cruise liners. The common problems of installing and using pod drive systems in pleasure craft are illustrative, however, to a greater or lesser degree, of the typically problems associated with using pod drive systems in all types of vessels and will be discussed below as examples of these problems.
FIGS. 2 through 6 are illustrations of various pod drive systems of the prior art as installed in a vessel having a V-bottomed planing hull with twin pod drive units mounted through the hull, as shown in FIGS. 2 through 4, at symmetrical positions on either side of the hull keel or centerline. Those of ordinary skill in the relevant arts will recognize, however, that such V-bottom hulls, and variations thereof, are commonly used on a variety of other vessels, including commercial and work craft, and vessels having rounded or curved bottoms will present similar problems because the pod drive units must be mounted on sections of the hull that are at an angle to both the vertical plane and the horizontal plane. It will also be recognized that at least some of the same or similar problems appear with flat bottomed hulls as well as will be apparent from the following discussion.
Referring again to FIGS. 2 and 3, a tunnel pod drive system 2 is shown therein as adopted, for example, by the Brunswick Corporation of Lake Forest, Ill. and described in U.S. Pat. Nos. 7,371,140 and 7,188,581 issued to Richard A. Davis for a Protective Marine Vessel and Drive and in European Patent Application Serial No. 1 777 154 A2 filed on Sep. 26, 2006 and published on Apr. 25, 2007.
As shown in FIGS. 2 and 3, but not in FIG. 4, the installation of twin pod drive units 2A in the V-bottom hull 4H requires the formation of corresponding open bottomed “tunnels” 4T, or canyons, on either side of the keel 4K with each pod drive unit 2A extending into a corresponding tunnel 4T through the top 4O of the tunnel 4T with underwater steerable gearcases 2E extending vertically below the tunnel top 4O and residing largely within the tunnels 4T. The propellers 2G are located partially within or extend largely below the bottom 4B of hull 4H and the steering axes 2L are generally oriented vertically. The forward ends of tunnels 4B are typically closed by a forward end wall 4F, for structural reasons, such as reducing the interior volume of hull 4H occupied by the tunnels 4T, while the aft ends 4R of tunnels 4T are open to permit the flow of water through the tunnels 4T and around the steerable gearcases 2E and the propellers 2G.
A primary advantage of a tunnel pod drive system 2, as illustrated in FIGS. 2 and 3, is that pod drive units 2A, and in particular steerable gearcases 2E and to a certain extent the propellers 2G, are better protected because pod drive units 2A are raised or recessed vertically, relative to the keel 4K, thereby at least partially protecting pod drive units 2A from striking an underwater object(s). Other possible advantages are that the navigational draft of the vessel is typically reduced allowing more water areas to be safely navigated by the vessel, and that steering by the thrust generating elements, that is the propellers 2G, generally allows greater manoeuverability and improved vessel handling characteristics.
However, a major disadvantage of a tunnel pod drive system 2, as illustrated in FIGS. 2 and 3, is the effect on hull characteristics caused by modifications to the hull to accommodate the tunnels 4T, particularly when an existing hull is modified for tunnel mounting of pod drive units 2. For example, the installation or provision of tunnels 4T not only results in significant structural changes to the hull but also reduces the amount of buoyancy of the vessel, toward the stern end thereof, thus reducing and/or redistributing the buoyancy of the vessel. The tunnels 4T have also been found to reduce the planing surface at the stern, thereby causing a “squatting” or “sinking” effect of the stern of the vessel that has been found to increase further in the event that the depth of tunnels 4T within the vessel is increased.
Other disadvantages are that the “wetted surface area” of the hull 4H is increased by the tunnels 4T, thereby increasing the frictional drag of hull 4H and correspondingly reducing the vessel speed while also increasing fuel consumption. The tunnels 4T have also been found to cause redirection of the flow of water around hull 4H, thereby further increasing the drag of the hull 4H. It has been found that the tunnels 4T may channel the flow of water, generated by the propellers 2G, thereby creating low pressure fields that result in a downward force, on the aft region of the hull, that may adversely effect vessel trim angles.
An alternate method for mounting pod drive units in twin engine V-bottom vessels is the slanted steering axis system 4 that has been adopted, for example, by the Volvo Penta system of Volvo Corporation of Greensboro, N.C. which is described, for example, in U.S. Pat. No. 7,033,234 issued to Arvidsson for Watercraft Swivel Drives and in U.S. Pat. No. 5,755,605 issued to Asberg for a Propeller Drive Unit, and in International Patent Applications WO96/00682 and WO96/00683.
As shown in isometric view in FIG. 4, the pod drive units 2A are mounted directly to hull 4H, in a slanted steering axis pod drive system 4, so that the steering axis 2L of each pod drive unit 2A is normal to the port and the starboard surfaces 4P and 4S of the hull 4H and is thereby at an angle to the vertical axis of the vessel.
A major advantage of the slanted steering axis pod drive system 4 is that the system does not require any tunnels 4T to adapt the pod drive units 2A to the hull 4H. The slanted axis system 4 thereby does not require any significant modification(s) to the shape or the structure of the hull 4H, does not effect or alter the buoyancy or distribution of the buoyancy or the trim of the hull, the fluid flow around the hull, the wetted surface area or the drag of the hull or some of the handling characteristics of the hull and, for example, does not result in low pressure areas in the aft regions of the hull with consequent “squatting” or “sinking” effects.
The pod drive units of FIG. 4 are, however, more exposed to damage in the slanted axis pod drive system 4, and the system typically results in the pod drive units, and thus the vessel, having an increased draft as compared to a tunnel mount system. Yet another aspect of the slanted steering axis pod drive system 4 is that, as can be seen from FIG. 4, the tilt of steering axes 2L—relative to a substantially vertical axis—results in each pod drive unit 2A producing a vertical component of thrust from the propeller 2G in addition to the horizontal component of thrust. The magnitude and direction of the vertical component of thrust, that is, either upward or downward, depends upon the direction and angle at which the propeller 2G is rotated and the slanted steering axis pod drive systems may be used, for example, to trim the running position of the vessel. That is, the pod drive units 2A may be rotated in opposite directions by an angle of rotation selected so that the horizontal components of the thrusts generated by the two pod units 2A cancel each other while the vertical components of the thrust, generated by each unit, is added to exert an upward or downward force on the stern of the vessel and to thereby adjust the fore/aft trim of the vessel to a desired setting or value. The rotations of the two pod drive units may be dynamically adjusted, in this way, to control the fore/aft trim of the vessel for various speeds or loading conditions, and may be used, for example, to adjust the fore/aft trim of the vessel during a transitory period, such as assisting the vessel over the planing threshold when transitioning from the displacement mode to the planing mode.
The generation of an upward or downward force on the vessel by a slanted steering axis drive system when the pod drive units are rotated is disadvantageous, however, because this effect often generates a “rolling” force and effect on the vessel during turns. That is, during a left or a right turn for example, the propellers 2G, of both pod drive units 2A, rotate about their steering axes 2L toward the left or right hand turn so that both pod drive units 2A exert a horizontal thrust component toward the inside of the turn, thereby forcing the stern toward the outside of the turn and forcing the vessel to turn in the desired direction. The rotation of the pod drive units 2A toward the inside of the turn, however, results in the vertical thrust generated by the inside pod drive unit 2A, that is, the pod drive unit 2A toward the inside of the turn, being directed downward while the vertical thrust component generated by the outside drive pod 2A is directed upward.
The combined vertical thrust components from the drive pod units 2A, in a slanted steering drive system 4 according to FIG. 4, thereby may exert a force during a turn that causes the vessel to have an unwanted rolling motion toward the inside of the turn. It has been found that this unwanted effect increases with the deadrise of the hull, that is, the angle of rise of the port and the starboard halves of the hull on either side of the keel. The rolling effect also places addition constraints on the center of gravity of the vessel because the center of gravity must be kept as low as possible to reduce excessive roll angles, during turns, and in the design of the transom because the height of the transom must be sufficient to accommodate the shift in the waterlines as the vessel rolls during turns.
Lastly, FIGS. 5 and 6 illustrate yet further embodiments of the pod drive systems. FIG. 5 is an isometric view of a single tunnel pod drive unit 2A installed in a tunnel 47 extending along the aft keel 4K of the hull 4H. It should be noted that, in FIG. 5, the pod drive unit 2A shown therein is a “tractor” propulsion unit. That is, the blade pitch of the propeller 2G and the orientation of the steerable gearcase 2E are reversed, with respect to the propellers 2G and the gearcases 2E illustrated in FIGS. 2 through 4, so the propeller 2G accordingly exerts a “pulling or traction” force on the vessel rather than the “pushing” force exerted by the propellers 2G and the gearcases 2E of the pod drive units 2A shown in FIGS. 2 through 4.
FIG. 6, in turn, is a rear view of the single tunnel pod drive system of FIG. 5 combined with the dual slanted steering axis pod drive system 4 of FIG. 4 to provide a triple pod drive system. It will be noted that in the illustrated combined pod drive system, the gearcase 2E and the propeller 2G are implemented as “pushing” units as shown in FIGS. 2 through 4, rather than a “tractor” or “pulling” unit as illustrated in FIG. 5. It will be understood, without further any discussion, that the system of FIG. 5 could also be combined with the system of FIGS. 2 and 3 to provide an alternate implementation comprising a triple tunnel pod drive system, providing either a pushing or a pulling force. It will be appreciated, however, that all such approaches to the problems of the pod drive systems of the prior art will generally have the same disadvantages as the embodiments illustrated in FIGS. 2 through 4.
The present invention is directed at addressing and overcoming the above noted problems as well as other problems associated with the known prior art systems.